I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for controlling transmission power in a variable rate communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech employs variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in copending U.S. patent application Ser. No. 08/004,484, which is a continuation application of U.S. patent application Ser. No. 07/713,661, filed Jun. 11, 1991, entitled "VARIABLE RATE VOCODER", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when said speech encoding is providing speech data at a maximum rate. When a variable rate speech coder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate is described in detail in copending U.S. patent application Ser. No. 08/171,146, which is a continuation application of U.S. patent application Ser. No. 07/822,164, filed Jan. 16, 1992, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. patent application Ser. No. 08/194,823, which is a continuation application of U.S. patent application Ser. No. 07/846,312, filed Mar. 5, 1992, entitled "DATA BURST RANDOMIZER", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In an alternative continuous transmission strategy, if the data rate is less than the predetermined maximum the data is repeated within the frame such that the data occupies the full capacity of the data frame. If such a strategy is employed, power consumption and interference to other users may be reduced during periods of data transmission at less than the predetermined maximum by reducing the power at which the frame is transmitted. This reduced transmission power is compensated by the redundancy in the data stream and can offer benefits in range for a fixed maximum transmission power.
In the exemplary embodiment, a variable rate communication signal is transmitted between a base station and a mobile station. The variable rate signal of the exemplary embodiment is transmitted at one of four possible rates. The rates are referred to as full rate, half rate, quarter rate and eighth rate. In systems where the transmission power of the signal is dependent upon the rate of the signal, a problem arises in determining the adequacy of the received variable rate signal.
One possible method for controlling transmission power in a communication system that varies transmission power in accordance with the rate of the signal being transmitted is for the system of the receiving end to send back to the transmitting device a bit indicating the adequacy of the received signal power for each possible rate. In a system that has four possible rates, this would entail transmitting four bits of power control information per power control group, where each bit corresponds to the adequacy of the received signal power for a different rate hypothesis. This power control method would present an unacceptable impact on system capacity. There is a need for a more efficient method of power control in continuous transmission variable rate communication systems.